oil pressure problem

[4VCLEVO]

I own an AU XR6 series 1 and its just ticked over 150000km. Over the last 3000km(aprox) the oil pressure has been very low when the engine is at full operating temp. At 100km/h the gauge sits between the to bottom white marks and at idle it goes into the red and brings on the low oil pressure light. When the engine is cold the oil pressure is great (reads half on the gauge), so I'm thinking the oil I'm using is now to thin at operating temp. The oil I have been using since 90000km is Valvoline durablend semi-synthetic 5w/30 which I change every 10000km and haven't had a problem until now. So what weight oil do people think I should now use, or do you think that the oil isn't the problem and something else is wrong.

[TE50 027]

This has come up a few times now. Have the oil pressure tested, most likely a bunged oil pressure sensor though. The series 1 also has a problem with an earth wire causing the fault you've described. Cost me $50 for half an hours labour to have a Ford dealer fix, (was done a couple of times before by different auto elecs but fault returned soon after, is all good now.)

[4VCLEVO]

I have changed the oil and put 10/40 semi-synthetic oil in it and the oil pressure is back to normal (gauge reads between half and the next line).